A coaxial cable is typically an excellent medium for transmitting video and data signals because it can resist electromagnetic interference (EMI) and can support high bandwidth transmissions. Coaxial cables commonly used today for transmission of RF signals, such as television signals, for example, typically include a core containing an inner conductor and a metallic sheath surrounding the core and serving as an outer conductor. A dielectric material surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In some types of coaxial cables, air is used as the dielectric material, and electrically insulating spacers are provided at spaced locations throughout the length of the cable for holding the inner conductor coaxially within the surrounding sheath. In other known coaxial cable constructions, an expanded foamed polymeric dielectric surrounds the inner conductor and fills the spaces between the inner conductor and the surrounding metallic sheath.
One important attribute of coaxial cable is its ability to propagate a signal with as little attenuation as possible. One method of measuring signal propagation is expressed as a percentage of the speed of light, commonly known as velocity of propagation (VP). Coaxial cables of the “air dielectric” type of construction have very good signal propagation characteristics, with VP values typically 90% or higher. However, these coaxial cables unfortunately have relatively limited bending characteristics and are typically susceptible to buckling, flattening or collapsing of the outer sheath, which may adversely affect the electrical properties of the cable and/or render it unusable. Consequently, air dielectric type coaxial cables typically require very careful handling during installation to avoid such damage. Additionally, they may not be recommended for use in installations requiring small radius bends or frequent reverse bends. Note that they typically have structural return loss (SRL) peaks corresponding to the disk separation distance (the disks being included along the length of the cable to support the conductor within the shield), which is usually designed to be above the standard operating bandwidth (i.e., greater than 1 GHz). Extended frequencies beyond 1 GHz can cause the disk spacing to become much closer, thus increasing Vp attenuation, and cost.
Coaxial cables of the “foam dielectric” type of construction, on the other hand, ordinarily possess significantly better bending properties than air dielectric cables. They can be more easily installed without undue concern over buckling, flattening or collapsing of the outer sheath and they can be used in environments where air dielectric type cables are unsuitable. However, they can be hampered by a somewhat lower velocity of propagation than air dielectric type cables. This reduction in VP and increase in attenuation loss may be attributable to the foam dielectric.
In addition, foam dielectric cables typically present more difficulties with attachment of the cable to connectors and couplings. “Connectorization” can be important to the operator making the connection, as on some occasions the connection process may be carried out in inconvenient locations (e.g., under a house or while perched on a telephone pole or the like). Ordinarily, prior to fitting a cable to a connector, a short length of the dielectric material is removed from the end of the cable with a coring tool, which leaves the core and shield of the cable free to contact and connect to the connector or coupling. It is important to remove a sufficient amount of the dielectric material from the core and sheath (i.e., leaving little to no dielectric material remaining) so that the electrical connection between the cable and the connector is sound. In most instances the dielectric material is bonded to the core and sheath with an adhesive, which can further complicate the coring process.
Other properties of coaxial cable that can be important to performance of the cable include usability across a wide bandwidth spectrum (at least 1.0 GHz, and perhaps as much as 5.0 GHz), good return loss characteristics (for example, at least −20 db, and preferably at least −30 db), flexibility, ruggedness and/or durability relative to the surrounding environment, and manufacturability. In particular, it would be desirable for a coaxial cable to have a wide usable bandwidth spectrum with acceptable attenuation characteristics that has a nominal 75Ω impedance (which is commonly used when attenuation characteristics are of primary concern) or 50Ω impedance (which is typically used when power transfer is of primary concern), and which meets at least some of the other performance characteristics of interest.